Our research efforts are directed toward investigating the effect and mode of action of hormones and xenobiotics on the regulation of enzyme patterns during perinatal development. Particular emphasis has been placed on the postnatal development of hepatic enzymes in the rat following exposure to testosterone propionate (TP), diethylstilbestrol (DES), and other hormonally active xenobiotics (polychlorinated biphenyls (PCB)). These enzyme markers are characterized by low catalytic during normal prepubertal life but increasing values in the adult rat resulting in enzyme levels which are significantly higher in one sex than the other. We report here the postnatal programming of five sex-dependent hepatic enzyme markers by TP and/or DES and the results of our studies on the role of the hypothalamic-pituitary-gonadal axis on regulation of some of these enzymes. We also report our findings on the effect of PCB gestational exposure on enzyme markers in these offsprings.